A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.
A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):
                    CD        =                              k            1                    *                      λ            NA                                              (        1        )            where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, k1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA or by decreasing the value of k1.
In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of 5-20 nm, for example within the range of 13-14 nm, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring.
EUV radiation may be produced using a plasma. A radiation system for producing EUV radiation may include a laser for exciting a fuel to provide the plasma, and a source collector module for containing the plasma. The plasma may be created, for example, by directing a laser beam at a fuel, such as droplets or particles of a suitable material (e.g., tin), or a stream of a suitable gas or vapor, such as Xe gas or Li vapor. The laser beam which is directed at the fuel is orientated perpendicular to the surface of the fuel so as to maximise the amount of energy that is absorbed by the fuel. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector. The radiation collector may be a mirrored normal incidence radiation collector, which receives the radiation and focuses the radiation into a beam. The source collector module may include an enclosing structure or chamber arranged to provide a vacuum environment to support the plasma. Such a radiation system is typically termed a laser produced plasma (LPP) source.
Using the LPP source described above to output radiation (such as EUV radiation) has been found to be fairly inefficient. One measure of the efficiency of an LPP source is its conversion efficiency (CE). One measure of CE is the ratio between energy radiated by the LPP source within a 2% bandwidth around the desired output wavelength, and the input energy supplied to the LPP source. The largest experimental values of CE the applicant has achieved with the LPP source described above operating to produce EUV radiation are less than 5%, and in some cases 3-3.5%.
One of the reasons using the LPP source is this inefficient is because there is poor coupling of the laser into the fuel. That is to say, the absorption of the laser radiation by the fuel is low. In some cases less than 50% of the laser radiation is absorbed by the fuel. Low absorption of the laser radiation by the fuel not only reduces the amount of output radiation produced by the plasma, but it also means that a significant proportion of the laser radiation is reflected or transmitted by the plasma. Laser radiation which is reflected or transmitted by the plasma may pass from the source collector module into portions of the lithographic apparatus which are downstream of the source collector module (such as the substrate). Laser radiation which passes from the source collector module into portions of the lithographic apparatus which are downstream of the source collector module may be referred to as out of band (OoB) radiation. OoB radiation may reduce the imaging performance of the lithographic apparatus and/or may lead to heating of portions of the lithographic apparatus which results in damage to the lithographic apparatus.